"Guanlong is the oldest and most primitive tyrannosaur," James Clark of George Washington University, who helped lead the study, told a news conference.

Tyrannosaurus Rex, the best-known tyrannosaur, lived 65 million to 70 million years ago, during the Cretaceous Period. Guanlong wucaii is 90 million years older.

"It is really pushing the record of tyrannosaurs back into the Jurassic (Period)," Clark said.

Clark's team found two examples of the early tyrannosaur in the Junggar Basin in northwestern China, a cold desert that is little explored.

Back in the Jurassic Period, the area would have been a large wetland bounded by mountains, said geologist David Eberth of the Royal Tyrell Museum in Alberta, Canada. It would have been warm, with sporadic volcanic explosions, he said.

"These animals were found in mud rock," Eberth told the news conference. He said the deposits were made of sand, clay and volcanic material that secured the fossilized bones for millennia.

The wind had now eroded the dried deposits of tens of millions of years. "It is very easy to see what is in the rocks, what is coming out of the rocks," Clark said.

The researchers camp out in the desolate region in the summers, working between sandstorms. They hire local people to walk across the rocks, systematically searching for protruding fossils.

SHARP TEETH

It was one such employee who spotted a bit of Guanlong sticking out and eventually the paleontologists dug up two nearly complete skeletons, one on top of another.

"We knew right away that this was a theropod dinosaur," Clark said. It had sharp teeth and clearly was a two-legged animal.

But one of the two specimens had an unusual, thin crest on its head, and there was a place on the second skull where one appeared to have been broken off.

"It's a very unusual skull with this crest coming off it," Clark said. "It doesn't immediately shout tyrannosaur when you first see it." But other identifiers marked it as a tyrannosaur, including distinctive structures in the pelvis.

The thin crest probably was used for display, Clark said. It has been colored red in the reconstruction, although scientists have no idea what color it would have been.

They have also given the animal bright purple feathers. "We have no evidence of feathers in this species," Clark said. "But there is evidence of feathers in another primitive tyrannosaur. We are fairly confident that it did have feathers."

Gregory Erickson of Florida State University studied the "growth rings" in the animals' shin bones and determined that one specimen would have been about seven years old and thus a juvenile, and the other a 12-year-old adult.

The creatures were about 10 feet (3 metres) long, compared to Tyrannosaurus Rex, which reached lengths of 42 feet (14 metres). They do not have T Rex's shortened forelimbs.